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Luxury London basement job puts Forcia and Geocisa in a tight spot

Work on a millionaire mansion near Harrods involving stringent party wall agreements and compact working conditions required clever solutions.

Project: 1 Ennismore Street
Client: London Capital Corp
Contract value: £1.3m
Geotechnical contact value: £0.2m
Region: London
Main contractor: Forcia
Groundworks subcontractor: Geocisa UK
Start date: October 2012
Completion date: November 2013

Installing Chelsea and Westminster basements has remained a bright spot for geotechnical firms with the millionaires that can afford them proving resilient to the recession.

Speaking to Construction News last autumn, Abbey Pynford chairman Paul Kiss said: “We predict a slight downturn in inward investment to London, but the basements market there will still remain strong. For underpinning, the market has been fairly constant.”

At 1 Ennismore Street, a short walk from Harrods, main contractor Forcia is rebuilding a two-storey private property with an attic for client London Capital Corp and installing a new two-level basement.

The £1.3m project is 99 per cent new-build and will retain only the two street-facing façades, while the basement will include a swimming pool and media room.

Lucrative basement work

Forcia site manager Ruben Farinas says this kind of investment appears to be paying off in this super-rich neighbourhood: “Surveyors are knocking on doors around here to see if properties can be bought for redevelopment,” he says. Basement developments are still a lucrative business.

“Using traditional piling could have reduced the available basement space by about 60 sq m across the two levels of basement, which in Westminster is a lot of money”

Ruben Farinas, Forcia

Due to the constraints of this site and consideration for local residents, the London-based main contractor decided against traditional piles.

“Using traditional piling could have reduced the available basement space by about 60 sq m across the two levels of basement, which in Westminster is a lot of money and would have also affected the design of the basements, making them too skinny,” Mr Farinas explains.

“It was important to not do this job with sheet piles or big piles, so we contacted Geocisa as a micropiling specialist. The client wanted to reduce noise and vibration disturbance to the neighbours, which ruled out traditional piles.”

Forcia also needed a reduced headroom solution, as in some instances it had only 3 m to play with.

The UK arm of the major Spanish geotechnical subcontractor is a relatively new entrant to the domestic market, making its debut in 2011 when it won work on Crossrail at both Farringdon and Canary Wharf stations.

Upheave protection

The contract in west London is for permeation grouting, micropiling and tension piling. The Forcia job has seen Geocisa secure the basement excavation from water ingress while protecting the structure from upheave.

Forcia installed the 3.5 m-deep, 1 m-long by 0.5 m-wide underpins around the structure’s perimeter in a standard A, C, B, D sequence of hit one, miss three.

Geocisa began work under a £230,000 design-and-build subcontract in February 2013, with its work ending in April.

“In some areas we use silicate grout to prevent heave, depending on how the ground behaves and whether it heaves or there are grout emissions”

Alejandro Segundo González, Geocisa

The permeation grouting consisted of installing an angled grout barrier beneath three of the four walls of the structure using PVC pipes with packers and sleeve ports known as tubes-a-manchettes. This sees PVC sleeves cast into the underpins to facilitate the TAM’s access.

Geocisa UK project manager Alejandro Segundo González says: “We are using tubes-a-manchettes with cement grout to prevent water ingress in to the basement.

“In some areas we use silicate grout to prevent heave, depending on how the ground behaves and whether it heaves or there are grout emissions [as the silicate behaves differently under pressure to grout].”

He explains that Geocisa made the first basement watertight, making it possible to put in the second level, as the water table starts at 3.9 m below Ennismore Street.

“There is 1.7 m of groundwater mixed with sands and gravels and sticky clay, with London Clay below,” Mr Farinas says. “Above this is 1.3 m of sand where Forcia can operate without water ingress, as it is above the water table.”

Ground control

Geocisa achieved watertightness using selective pressure grouting and TAMs to convey different pre-assigned quantities of grout at varying pressures. But these requirements can change during pumping depending on how the ground responds.

The trick is to prevent the grout from each TAM creating pressure against that of previously installed adjacent ones.

To ensure a seal against groundwater, the grout seal extends down 1 m into the impermeable London Clay.

Geocisa used its experience to determine whether more was required to secure the perimeter against water ingress. In other cases less was needed, as the team achieved a seal more quickly than expected and so operations stopped to protect the building from excessive pumping creating a heave reaction.

Party wall agreement affects method

The micropiling work will produce a vertical curtain retaining wall for the west end of the site in the terrace gravel layer.

“Virtually no water comes in from the outside now the grouting is finished”

Ruben Farinas, Forcia

The owner of this adjoining wall stipulated in the party wall agreement that it forbids grouting below its next-door property.

Geocisa put in these micropiles using selective pressure grouting through the terrace gravel layers, in a broadly similar way as the permeation grout barrier of the TAMs.

But the vertical installations are at 250 mm centres and 500 mm from the wall, and millimetres can soon become a lot of money in this part of the capital.

Tension micropiles stop structure floating

The tension micropiles, installed using non-selective pressure grouting, anchor the base-slab and resist uplift forces.

Geocisa put these in from the first basement level and through the second unexcavated level, with the micropiles founding at the bottom of the second level. Forcia then removed the upper portion of the micropile as it excavated down.

At the north wall opposite the street, the angled TAM tubes penetrated beneath the adjoining properties beneath existing underpins, which were installed about one year earlier for a basement beneath that adjacent property.

In the early stages of the construction, Forcia had to pump out or drain water from some trial pits excavated on site because the pressure of the ground water made any work impossible below the table level.

But since the TAM work completed, the grout’s effectiveness is clear. “Virtually none comes in from the outside now the grouting is finished,” Mr Farinas says.

As basement work advanced, Forcia was able to simultaneously rebuild the rest of the above-ground building.

At its completion date in November, the client London Capital Corp will have a highly desirable five-level property in one of the world’s most expensive neighbourhoods.

The underpinning sequence

Working from ground level, Forcia first excavated a 1.1 m-long by 1.5 m-wide pit inside the building, adjacent to the wall, so it could get access underneath to undermine the existing foundations and install underpins.

Each 1 m-deep pit section received 2-inch timber shutters propped with steel struts to hold the face of each excavation, which were also fitted with a mesh to prevent any soil that does come loose from interfering with work.

Site workers then shuttered the face of the forthcoming 3.5 m-deep underpin and poured concrete, striking the shutters 24 hours later.

They then backfilled the pit and compacted in 1 m layers, removing props as they went, before starting on the next pit in the hit one, miss three sequence.

After all the first basement level underpins had been installed the excavation could begin, with support from temporary horizontal whaling beams, allowing Geocisa’s Klemm rig access to pump in the grout curtain.

With the curtain intact, Forcia excavated the second level of basement, repeating the process used for the first.

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